1. Field of the Invention
The present invention relates to a method and an apparatus for controlling internal combustion engines equipped with an electronically controlled fuel injection system and more particularly to a method and an apparatus for controlling internal combustion engines, which employ electronic control circuitry to control fuel injection quantity, ignition timing, etc., so as to stably control engine rotational speeds at the time of idling and a low speed operation of the engines.
2. Description of the Prior Art
In a known type of internal combustion engine (hereinafter simply referred to as an engine) equipped with a speed-density type electronic fuel injection system, a fuel injection quantity has been determined in such a manner that the basic fuel injection quantities given by a two-dimensional map in accordance with engine speeds and intake pipe pressures, or the basic fuel injection quantities obtained by applying engine speed compensation to the fuel injection quantities determined in accordance with intake pipe pressures determine a value which substantially satisfies a stoichiometric air-fuel ratio, and such basic fuel injection quantities are further compensated for variations in the cooling water temperature, intake air temperature, battery voltage, etc., thereby providing a resultant controlled fuel injection quantity.
However, the above-mentioned basic fuel quantity is determined principally from the value of the intake pipe pressure, and the effect of the engine speed thereon is small as compared with that of the intake pipe pressure.
If any disturbance is applied to an engine operating under no load condition, not only the engine speed and the intake pipe pressure are varied, but also the fuel injection quantity is varied substantially in phase with the variation of the intake pipe pressure for the above-mentioned reason. However, in the case of an engine equipped with a speed-density type electronic fuel injection system, the intake system has a large-capacity surge tank and this gives rise to a phase difference between the engine speed and the intake pipe pressure. Consequently, a phase difference appears between the engine speed and the fuel injection quantity. As a result, if the engine speed decreases, the air-fuel ratio becomes leaner and the torque decreases, which, in turn, further decreases the engine speed. On the contrary, if the engine speed increases, the air-fuel ratio becomes richer and the torque increases, and this results in a further increase in the engine speed. Thus, there is involved a disadvantage that the variation of the engine speed is enhanced and the engine speed becomes unstable.
To overcome the foregoing disadvantage, it has been proposed to adjust the air-fuel ratio characteristic around the idling operation on the basis of a certain predetermined intake pipe pressure (an average idling intake pipe pressure of a large number of engines) in such a manner that when the engine speed becomes higher than a predetermined idling speed and the intake pipe pressure becomes lower than the predetermined intake pipe pressure, the basic fuel injection quantity is compensated to enrich the air-fuel mixture, whereas when the intake pipe pressure becomes higher than the predetermined intake pipe pressure, the compensation is effected to make the air-fuel mixture leaner. However, even if the basic fuel injection quantity is compensated on the basis of such a predetermined intake pipe pressure in such a manner that the mixture is enriched when the intake pipe pressure becomes lower than the predetermined pressure and the mixture is made leaner when the reverse is the case, the intake pipe pressure during an idling operation differs for every engine due to variations in performance of the respective engines. Therefore, it is impossible to expect an identical functional effect on all manufactured engines when they are put on the market. In addition, after the engines have been put to practical use the intake pipe pressure during an idling operation varies due to the wear and the secular variations of idling air flow, with the resultant deterioration of the stability of the idling operation and the exhaust emission. Further, with vehicles of the type employing an exhaust emission control system comprising an oxygen concentration sensor feedback system including a three-way catalyzer, even if the basic fuel injection quantity is compensated for variations in the intake pipe pressure with respect to the previously mentioned predetermined intake pipe pressure, the stability of an idling operation will be deteriorated considerably due to variations of the air-fuel ratio caused by the feedback action. Further, if the capacity of the surge tank is increased to increase the engine output, the phase difference between the engine speed and the fuel injection quantity will also be increased thus making the engine speed unstable, and after all making it practically impossible for the prior art methods to overcome these defects.
The present invention has been made in view of the foregoing defects involved in the prior art.